I. Field of the Invention
The present invention relates to the removal of sulphur and sulphur dioxide components from gas streams, and more specifically to the removal of these and other similar elements from stack gasses and smelter gasses of the type produced from the combustion of hydrocarbons such as coal, etc.
II. Description of the Prior Art
As the cost of petroleum products continues to escalate, many energy intensive industries, and especially electrical generating plants, have investigated the costs required in switching from oil-burning boilers to coal-burning boilers. Since the coal may be obtained from within the continental boundaries of the United States at a much lower price than petroleum distillates, and since the supply of coal is much more reliable than off-shore petroleum supplies, the United States Government has indicated a preference for the use of coal for such combustion requirements consistent with maintaining clean-air standards and other ecological requirements of the surrounding area. However, one of the main problems involved in converting from petroleum combustion to coal combustion boilers is that much of the remaining United States coal supplies contains large amounts of sulphur. During the combustion process this sulphur will combine with available oxygen to produce sulphur dioxide and other similar components. Other undesirable by-products such as nitrogen dioxide and carbon monoxide are produced by insufficient and imperfect combustion.
Many inventors have attempted to eliminate these effluents from stack gasses produced by electrical generating boiler combustion processes. However, present technology has drifted toward the use of gas scrubbers and other various systems which are extremely expensive to construct, install and operate. Other similar systems employ expensive designs such as the use of catalysts for enhancing the chemical process or the use of secondary combustion processes.
Various prior art references are available for evaluating the state of technology in this field. Palilla, in U.S. Pat. No. 3,931,390, discloses a catalytic process for reducing sulphur dioxide in gas streams containing sulphur dioxide to elemental sulphur components using a reducing gas such as hydrogen or preferably carbon monoxide together with a catalyst chosen from Ln.sub.2 O.sub.3, Co.sub.2 O.sub.3, where L.sub.n is either Y or Gd. These catalysts are used to enhance the chemical reaction between the elements and compounds existing within the exhaust gas effluent in order to produce elemental sulphur and other contaminents from the exhaust gas effluent.
Hass, in U.S. Pat. No. 4,060,589, discloses a process whereby No.sub.x and SO.sub.x components contained in stacked gasses are simultaneously reduced to N.sub.2, H.sub.2 and S respectively by passing the stack gasses, which also contain steam and usually some oxygen, over bituminous coke at temperatures between about 900.degree. and 1600.degree. F. The coke effects the simultaneous reduction of SO.sub.x and NO.sub.x components as well as filtering particulate matter from the stack gasses. The coke also produces sufficient hydrogen during the reduction of SO.sub.x and NO.sub.x components such that when the purified gas stream is passed over a conventional hydrogenation catalyst, any residual SO.sub.2 present therein will be reduced.
Ehlig, in U.S. Pat. No. 3,865,929, discloses a process whereby a crude hydrogen fluoride gas is fed into a gas cooler. The gas is cooled by evaporating a recirculating liquid stream containing the liquid hydrogen fluoride. This liquid stream removes sulphur impurities from the crude gas. The recirculating stream is recycled through the gas cooler a sufficient number of times until the sulphur impurities accumulate to a particle size that is easily removed by conventional liquid-solid separating techniques such as filtration. Portions of the cooled gas may be removed and subsequently condensed to form crude liquid hydrogen fluoride.
Hammond, in U.S. Pat. No. 3,760,061, discloses a process whereby sulphur dioxide may be oxidized to sulphuric acid by a process that involves contacting the SO.sub.2 with an aqueous scrubbing solution of sulphuric acid containing hydrogen peroxide and/or the peroxy acids of sulphur in a gas-liquid contacting device in which the liquid scrubbing phase is made the continuous phase during the actual contacting.
Brown, in U.S. Pat. No. 3,635,000, discloses a process whereby suspended particulate matter, including sulphur-containing compounds of calcium and magnesium, and other sulphur oxides are removed from combustion gasses by passing the gasses successively through a high-velocity venturi-type scrubber and a wetted film-packed bed scrubber. The scrubbing liquid from the venturi scrubber and from the packed bed scrubber may, after being clarified, be recirculated to the two scrubbing stages.
The prior art also includes the teaching of Tolles, in U.S. Pat. No. 3,862,295, Jones, in U.S. Pat. No. 3,715,871, Von Linde in U.S. Pat. No. 2,843,217, Crowley, in U.S. Pat. No. 3,733,788, Saleem, in U.S. Pat. No. 3,907,526, Yamamichi, in U.S. Pat. No. 3,943,230, Paraskezakos, in U.S. Pat. No. 3,812,296, and Teller in U.S. Pat. No. 3,968,482. References also made to various articles relating to stack gas processing which are included in pages 41 through 80 of the February 1978, issue of CEP MAGAZINE.